Friday, 3 April 2009

Brief intro to LINQ

Ive recently given a breif tutorial to the guys on our team on LINQ (Language integrated query)

As with Generics, LINQ is a large subject that i cant write up properly without spending quite a lareg amount of time on, so i'll give a breif overview with an example of a common usage in the context of a generic list.

Overview

LINQ is a technology (or language construct) that allows you to process data using functional programming logic to perform an operation. The LINQ API is an attempt to provide a consistent programming model that allows you to manipulate data. Using LINQ we are able to create directly within the C# programming language entities called query expressions, these query expressions are created from numerous query operators that have been intentionally designed to look and feel similar (but not identical) like SQL. Using LINQ you can interact with many sources of data, listed below:

LINQ implementations
LINQ to objects
Linq to SQL
Linq to entities
Linq to XML
There are other LINQ enabled technologies but these are the most common, for a grater list look online.

All of this means that you can interact with an XML dom in the same manner as you would with a list of objects or an ADO record set without having to learn different syntax for either.

LINQ Example

List names = new List(){"George", "William Smith", "bob", "Fred"};
IEnumerable subSetNames = from n in names
where n.Length >= 4
order by n
select n;
foreach(string name in subSetNames)
{
Console.Write(name);
}

The code above firstly creates a list of strings and places then in the names variable
Then it takes that collection filters it where the length is greater than or equal to 4
Sorts it based on the string
Puts the resulting collection in the variable subSetNames?
Finally it loops through all the strings in subSetNames? and prints them out

The resulting string is "Fred George William Smith" In this example the collection being acted on was a simple list of strings but it could be a collection of any type of object, or indeed an ADO record set, or an XML document, its a very powerful technique and allows for very clear functional logic (no complex nested loops and temporary collections for ordering etc).

This topic is massive and so im not going to go into it in any more depth, but in the most part ive not used any LINQ that is anymore complex than the example listed above. more advanced features such as lambdas and extension methods start to make things complex and ive so far managed to keep clear of using these techniques, for more info search google for the answer.

LINQ Grouping

By way of an example this code below demonstrates the power of LINQ used with a simple grouping command

string[] names = {"Albert", "Burke", "Connor", "David", "Everett", "Frank", "George", "Harris"};

//Defining the GroupBy logic (here it is lengthwise)
var groups = names.GroupBy(s => s.Length);

//Iterate through the collection created by the Grouping
foreach(IGrouping group in groups)
{
//Branch on the condition decided
Console.WriteLine("Strings of Length {0}", group.Key);
//Actual results
foreach(string value in group)
Console.WriteLine(" {0}", value);
}


Produces this output

Strings of Length 6
Albert
Connor
George
Harris
Strings of Length 5
Burke
David
Frank
Strings of Length 7
Everett

LINQ Resources
MSDN 101 LINQ samples

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